In recent developments in medical diagnosis and examination, a technique known as nuclear magnetic resonance (NMR) imaging has come into use in neurological diagnosis. Techniques related to NMR imaging have been used for chemical diagnosis.
Such NMR systems conventionally comprise a large solenoidal magnet which provides a uniform magnetic field surrounding the subject, a radio-frequency (RF) coil surrounding the subject matter and a receiver coil surrounding the subject. As the size of the systems has increased, utilizing large magnets which are super cooled producing magnetic fields on the order of 10 kilogauss, significant problems have arisen with respect to shielding the magnetic field so that it does not affect persons or devices in the proximity. Thus, in order to prevent such adverse affects, it has become common to isolate the building containing the NMR system or to shield the walls of the buildings with appropriate shielding panels.
Accordingly, among the objectives of the present invention are to provide an NMR system which will effectively confine the magnetic field of the NMR system without affecting the field to any great extent, which maintain the central field at a large magnitude and which will not adversely affect the environment such that no additional protection is required for the system. In addition, an objective of the present invention is to preserve the uniformity of the high field in the center of the magnet. The shield has the effect of screening out the perturbing effects of large masses of magnetic material near the system magnet.
In accordance with the invention, a cylindrical shell of magnetic material surrounds the magnet and the RF and receiver coils. The shell is positioned such that the wall thereof lies in the area of the field of the magnet where permeability is substantially at a maximum in the shell material for the imposed field such that the shell functions to substantially confine the field of the magnet to an area about the shell and the environment is not adversely affected and no additional protection is required surrounding the system.